Ants
- Some species of Australian ants (Hymenoptera:
Formicidae) are important predators of cotton pests, particularly under
raingrown conditions and in lighter soils where ant colonies can
establish within the field. However ants can sometimes form
associations with aphids, in which they feed on aphid honeydew, and in
return, nurture and defend aphid colonies against other predators.
During an
irrigation event, ants present in the crop will re-locate their eggs higher into the plant canopy.
Bacteria
- Bacterial disease may also affect larvae of
various cotton pests such as heliothis. Commonly, bacterial infections
can be observed after artificial infection with Bacillus thuringiensis
(Berliner), sold commercially as Bt sprays.
Bacterial Blight (Xanthomonas Axonopodis or X. Campestris Pvmavacearum - Exotic Strains)
- Although
strains of bacterial blight are alreadypresent in Australia, they are
no longer a problemdue to varietal resistance. Exotic strains
(races)occur, however, that are ‘hypervirulent' and, ifestablished in
Australia, would cause large yieldlosses. The disease is seed borne
allowing easydispersal and introduction of new races into newareas.
Bacterial blight is spread by high temperature,humidity and rainfall.
The initial symptoms include the undersides of leaveshave angular
water soaked lesions. Lesions dry anddarken with age then leaves are
shed. Black lesionsspread along stem. Bolls often infected at base or
tip.Lesions dry out and prevent the boll opening. The pathogen is
capable of symptomless
transfer
andtherefore could be undetected through quarantine.Symptoms include
yellowing or bronzing of leaves,leaves wilt and die; dead leaves usually
remain onplant. At this stage, roots are dead and surface iscovered
with
network of tan fungal strands.
Fungal Diseases
- Fungal diseases may affect larvae of
heliothis, cotton looper, cluster caterpillar and armyworms. Prolonged,
moist conditions favour the development of fungal diseases such as
those caused by Nomuraea rileyi (Farlow)Samson and Beauveria bassiana
(Balsamo) Vuillemin. Larvae infected by fungus become stiff and are
anchored to the plant by fungal threads. Later, they are completely
covered in white fungus which produces a velvety layer of spores under
moist conditions, green in the case of N. rileyi or white for Beauveria
spp. These spores infect other larvae and other species of insects
producing fungal threads which penetrate their bodies.
TExAS Root Rot (Phymatotrichopsisomnivore)
- Texas
root rot is an extremely damaging fungal diseasewith a wide host range.
It causes sudden death ofaffected plants, usually during the warmer
months. Incotton, infection can result in 100% crop loss. If
thisdisease became established in Australia, control wouldbe extremely
difficult as management using rotationsand fungicides is usually only
partially effective.Symptoms include yellowing or bronzing of
leaves,leaves wilt and die; dead leaves usually remain onplant. At this
stage, roots are dead and surface iscovered with network of tan fungal
strands.
Fusarium Wilt (Fusarium oxysporum F. sp. vasinfectum - Exotic Strains)
- Fusarium
wilt is a fungal disease. Strains ofFusarium were identified in
Australia in 1993however the introduction of new strains (races)would
increase the difficulty of management as newresistant varieties would
be required.External symptoms can appear in the crop atany stage but
most commonly appear in eitherthe seedling phase or after flowering
when bollsare filling. Leaves appear dull and wilted beforeyellowing or
browning progresses to eventual deathfrom the top of the plant.
Seedlings may either wiltand die or survive, but often with stunted
growth.Adult plants may wilt and die, especially underconditions of
stress. Some affected plants mayre-shoot from the base of the stem.
Lengthwisecutting of the stem from affected plants will showcontinuous
brown discolouration of the tissue.The internal discolouration is
similar to that ofVerticillium wilt but usually appears as
continuousbrowning rather than flecks. Sometimes thediscolouration is
visible in only one side of thestem. External symptoms do not always
reflect theextent of discolouration in the stem.
Blue Disease
- Blue disease is a virus specific to
cotton that canreduce yield potential by up to 20%. It is spread bya
vector, the cotton aphid. It has been associatedwith plants infected
with cotton leaf roll dwarf virus(CLRDV) and has similarities with
cotton bunchytop, anthocyanosis and cotton leaf roll. It is notknown if
the same pathogen causes all these diseasesor if there are multiple
pathogens causing similarsymptoms. CLRDV was not detected from
Australiancotton affected by cotton bunchy top disease. Cottonblue
disease affected leaves tend to be smaller, thick,more brittle and
leathery and have an intense greento bluish colour with yellow veins.
Reddening of stempetioles and leaf veins can occur in some
infections.Leaf edges tend to roll downwards and under andplants become
stunted due to a shortening of thebranch internodes and produce many
branches,giving a bunchy zig-zag stem habit. Symptoms aremore obvious
in plants infected at an early age andstunting is more pronounced.
Infected plants alsoproduce smaller bolls and boll shed may occur.
Singleinfected plants can be overlooked if overgrown bynearby healthy
plants.
Cotton Leaf Curl Disease (CLCUD)
- CLCuD, sometimes
referred to as Gemini virus, cancause yield losses of up to 35% in
cotton. It is spreadby a whitefly vector. There are at least seven
differentbegomoviruses and several different DNA satellitemolecules
associated with CLCuD. A cotton plantneeds to be infected with at least
one begomovirusand one satellite to develop CLCuD.Symptoms of CLCuD
are seen on leaves andinitially appear as a swelling and darkening of
leafveins, followed by a deep downward cupping of theyoungest leaves
then either an upward or downwardcurling of the leaf margins. Leaf-like
structures(enations) on the veins are common and vary in sizefrom only
a few millimetres in diameter to almostthe size of a normal leaf.
These larger structures areoften cup-shaped.
Indian Green Jassid (Amrascadevestans)
Indian green jassid is a sap-sucking
insect pest thatcan cause yield losses of up to 25%. While several‘jassid' species are found in Australian
cotton
.The damage they cause is relatively minor, rarely if everaffecting
yield. Green jassids inject a toxin as they feed that causes leaves and
bolls to drop and canstunt plant growth. Elsewhere green jassids can
bemanaged using resistant varieties and insecticides.Hairy-leafed
varieties are used in parts of Africa andthe sub-continent where cotton
is hand harvestedto provide effective resistance against green
jassids.Such varieties are not suitable for mechanical harvestas the
leaf hairs cause excessive leaf trash in thecotton lint.
Bugs
Bigeyed Bug
- The huge eyes on the bigeyed
bug, Geocoris lubra Kirkaldy provide easy identification. The
cylindrical, white eggs are laid singly on terminals and under leaves.
Adults and nymphs are fast-moving, daytime hunters of a range of
soft-bodied insects and mites which they kill with piercing/sucking
mouthparts. Numbers in cotton are highest during flowering. Germalus
sp. is a slightly larger, related species also common in cotton but the
two may be distinguished by colour. Geocoris is dark brown to black
whereas Germalus is brown and green.
Brown Smudge Bug
- The habits of the brown smudge
bug, Deraeocoris signatus (Distant) are not well known, though it has
been found to feed on heliothis eggs, aphids and the apple dimpling
bug. The maroon coloured nymphs and the adults are very aggressive
predators of mites. It is most abundant in unsprayed cotton in
December, January and February.
Damsel Bug
The damsel bug, Nabis kinbergii Reuter inserts
its eggs singly into soft plant tissues so that the circular emergence
caps protrude above the surface. Nymphs are small and wingless but
otherwise similar to the adults. Nymphs and adults use piercing/sucking
'beaks' to feed on many insects including eggs and larvae of moths,
mites and aphids. They are usually present in any kind of crop infested
with caterpillars. Maximum numbers occur in unsprayed cotton at the
same time as maximum numbers of heliothis larvae. Adult damsel bugs may
be found on weeds and crops during winter.
Glossy Shield Bug
- Larvae including those of loopers and
heliothis are the usual food of the glossy shield bug, Cermatulus
nasalis (Westwood) and its nymphs. The black eggs which are laid in
'rafts' of 50 or more have short, white spines around the rim. Nymphs
are dark red and brown with the early instars being bright red. Maximum
numbers are found in unsprayed cotton during midsummer. In early
summer they can be found in linseed and lucerne crops infested with
heliothis. This species is easily confused with the brown shield bug.
Pirate Bugs
- Pirate bugs, Orius spp. prey on thrips
and eggs of other insect pests in cotton. They often appear in large
numbers at flowering when thrips are in abundance.
Predatory Shield Bug
- The predatory shield bug,
Oechalia schellenbergii (Guerin-Meneville) is another species which
feeds with piercing/sucking mouthparts mainly on caterpillars. Adults
can be found at any time of year on weeds or crops infested with
caterpillars. The black eggs are laid in 'rafts' usually in multiples
of 14, with long, white spines around the rim. The nymphs are dark red
and brown. Adults and nymphs are usually most abundant in cotton from
December to February.
-
Spiders
- Flower or crab spidersFlower or crab spiders,
(family Thomisidae) have crab-like legs and are often brightly
coloured. They specialise in hiding on flowers where they are well
camouflaged and ambush small insects attracted to nectar and pollen.
They make no web but females cover their eggs with a silken shelter,
usually under leaves.
Jumping Spiders
- Jumping spiders (family Salticidae)
have two particularly large, forward-facing eyes which give them
excellent binocular vision. They rest during the night and actively
roam plants during the day jumping on prey detected by eyesight. They
are generalist predators but have been observed eating jassids and
small heliothis larvae. Except for the first pair, the legs are
relatively short. Most species on cotton are predominantly dark or grey
in colour.
Lynx Spiders
- Lynx spiders, Oxyopes spp. are active
hunters usually seen near the tops of plants. They do not make a web
but use their good eyesight to locate prey, then jump on it from a
distance. Their legs bear numerous characteristic spines. Their
abdomens are narrow and pointed and usually pale brown to green and
striped. As in almost all spiders, the young disperse by 'ballooning'
from tall plants where the silk is played out into the breeze until the
pull lifts and carries them away. Most spiders arrive on cotton in
this way in early summer and do not mature and reproduce until the crop
is flowering. Lynx spiders are generalist predators like most spiders
and have been observed eating small helicoverpa larvae and jassids and
are known as important predators of Mirids (Cotton Flea hopper) in the
United States.
Nightstalking Spiders
Small nightstalking spiders,
Cheiracanthium spp. are pale yellow with a faint grey mark on the
abdomen. The 'head' of larger specimens may become pale orange (yellow
nightstalking sac spider - Cheiracanthium mordax L. Koch). During the
day they hide in silken retreats, often under leaves and during the
night they stalk over plants searching for slow-moving prey. They will
eat heliothis eggs as well as larvae. Mating takes place in midsummer
in large silk retreats and the females guard their eggs in the retreats
until they hatch.